1. Field of the Invention
The present invention relates to a method for producing a solder containing additive particles therein.
2. Description of Related Art
In the prior art, a minute amount of copper has been added into a solder in order to improve a strength and creep characteristics.
As a solder material for bonding a semiconductor chip on a bonding substrate, there has been developed a solder for soldering between parts in such a manner that the shape of additive particles is maintained as it is and therefore a desired spacing is uniformly ensured between the parts by action of the additive particles (For example, JP-A-56-014069).
In order to produce a solder containing therein this type of additive particles, as shown in FIG. 1, additive particles 5 are added from an additive particle supplying vessel 7 into a molten solder material 1 within a molten solder bath 9, and the solder material 1 and the additive particles 5 are sufficiently stirred and mixed at a high temperature.
Thereafter, a molten solder material 1 mixed with the additive particles 5 is teemed from an opening of the molten solder bath 9, and caused to pass a cooling tunnel 6 so that the molten solder material 1 is cooled and solidified. Then, the solidified solder material is rolled by a pair of rollers 2 to a solder material sheet 3 having a predetermined thickness, and further, the solder material sheet 3 is cut into a plurality of narrow solder strips having a predetermined width. Finally, each of the narrow solder strips is wound on a reel 4 by a predetermined length.
In the conventional method as mentioned above, however, in the case of the additive particles formed of a low melt point metal such as Cu powder and Cu/Ag coated powder, the additive particles 5 are molten into the molten solder within the high temperature molten solder bath 9 in the course of the stirring and mixing process, and therefore, the additive particles cannot keep their original shape. In addition, in the case of the additive particles 5 such as Ni powder, Mo powder, W powder, glass beads, BN powder, etc, having a specific gravity lighter than that of the solder material, the additive particles are separated from and floated on the solder material 1 within the molten solder bath 9. On the other hand, in the case of the additive particles 5 having a specific gravity heavier than that of the solder material, the additive particles settle within the molten solder bath 9. In both cases, the distribution of the additive particles 5 within the solder material 1 becomes nonhomogeneous. For example, the content of the additive particles in the finished solder has varied in the range of 0 weight % to 0.5 weight %.
This solder is not suitable as a material for bonding between the semiconductor chip and the die bonding substrate. For example, not only it becomes difficult to control a solder layer to a desired thickness, but also the thickness of the solder layer directly under the semiconductor chip becomes uneven, and therefore, the semiconductor chip is bonded on the bonding substrate in an inclined condition. For example, the unevenness in thickness of the solder layer on the die bonding substrate has reached 30 .mu.m at maximum. As a result, an adverse influence affects a semiconductor chip recognition in the case of a wire bonding.